Delphi is a programming language developed by Borland in 1994, as a successor to Turbo Pascal. Development of Delphi was started in 1993 and its first version, Delphi 1.0, was launched on 15 February 1995.
Xamarin is a cross-platform development platform that allows developers to write mobile applications in C#, and run them on major mobile platforms (Android and iOS).
In February 2016, Xamarin was acquired by Microsoft, which means that Xamarin is now integrated with Microsoft’s developer tools and services. Alongside this, Xamarin SDK also became open source.
Qt is a cross-platform framework used widely for app software development. It can be run on a multitude of software and hardware platforms, such as: Windows, macOS, Linux, Android, iOS and more.
Although Qt is especially used for developing apps with graphical user interfaces, it can also be used for developing apps that don’t have a user interface.